The Two-Loop Anomalous Dimension Matrix

for Soft Gluon Exchange

S. M. Aybat, L.D., G. Sterman hep-ph/0606254, 0607309

Workshop HP2, ETH Zürich September 6-8, 2006

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 2

Outline

• Separation of QCD amplitudes into soft, collinear (jet) and hard functions• Computation of soft anomalous dimension matrix via eikonal (Wilson) lines to two loops (NNLL)• Proportionality of one- and two-loop matrices• Consistency of result with 1/ poles in explicit two-loop QCD amplitudes• Proportionality at three loops?• Implications for resummation at NNLL

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 3

IR Structure of QCD Amplitudes [Massless Gauge Theory Amplitudes]• Expand multi-loop amplitudes in d=4-2around d=4 (=0) • Overlapping soft (1/) + collinear (1/) divergences at each loop order imply leading poles are ~ 1/2L at L loops

• Pole terms are predictable, due to soft/collinear factorization and exponentiation, in terms of acollection of constants (anomalous dimensions)

Mueller (1979); Akhoury (1979); Collins (1980), hep-ph/0312336; Sen (1981, 1983); Sterman (1987); Botts, Sterman (1989); Catani, Trentadue (1989); Korchemsky (1989); Magnea, Sterman (1990); Korchemsky, Marchesini, hep-ph/9210281; Giele, Glover (1992); Kunszt, Signer, Trócsányi, hep-ph/9401294; Kidonakis, Oderda, Sterman, hep-ph/9801268, 9803241; Catani, hep-ph/9802439; Dasgupta, Salam, hep-ph/0104277; Sterman, Tejeda-Yeomans, hep-ph/0210130; Bonciani, Catani, Mangano, Nason, hep-ph/0307035; Banfi, Salam, Zanderighi, hep-ph/0407287; Jantzen, Kühn, Penin, Smirnov, hep-ph/0509157

• Same constants control resummation of large logarithms near kinematic boundaries

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 4

Soft/Collinear FactorizationMagnea, Sterman (1990) Sterman, Tejeda-Yeomans, hep-ph/0210130

• S = soft function (only depends on color of ith particle; matrix in “color space”)• J = jet function (color-diagonal; depends on ith spin) • H = hard remainder function (finite as ; vector in color space) color: Catani, Seymour, hep-ph/9605323; Catani, hep-ph/9802439

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 5

• For the case n=2, gg 1 or qq 1,the color structure is trivial, so the soft function S = 1

• Thus the jet function is the square-root of the Sudakov form factor (up to finite terms):

_

The Sudakov form factor

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 6

Jet function

Pure counterterm (series of 1/ poles);like (,s), single poles in determine K completely

• By analyzing structure of soft/collinear terms in axial gauge, find differential equation for jet function J[i] (~ Sudakov form factor):

finite as contains all Q2dependence

Mueller (1979); Collins (1980);Sen (1981); Korchemsky, Radyushkin (1987); Korchemsky (1989); Magnea, Sterman (1990)

K, G also obey differential equations (ren. group): cusp anomalous dimension

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 7

can be extracted from fixed-order calculations of form factors or related objects

• Solution to differential equations

s = running coupling in D=4-2_

E.g. at three loops Moch, Vermaseren, Vogt, hep-ph/0507039, hep-ph/0508055

Magnea, Sterman (1990)

Jet function solution

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 8

Soft function

• For generic processes, need soft function S• Less well-studied than J• Also obeys a (matrix) differential equation:

Kidonakis, Oderda, Sterman, hep-ph/9803241

soft anomalous dimension matrix

Solution is a path-ordered exponential:

For fixed-angle scattering with hard-scale let momenta with massless 4-velocities[ matches eikonal computation to partonic one]

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 9

• Only soft gluons involved• Couplings classical, spin-independent • Take hard external partons to be scalars• Expand vertices and propagators

More formally, consider web function W or eikonal amplitude of n Wilson lines

E.g. for n=4, 1 + 2 3 + 4:

Computation of soft anomalous dimensions

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 10

Soft computation (cont.)• Regularize collinear divergences by removing Sudakov-type factors (in eikonal approximation), from web function, defining soft function S by:

• Soft anomalous dimension matrix determinedby single ultraviolet poles in of S:

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 11

1-loop soft anomalous dim. matrix1/ poles in 1-loop graphyield:

Kidonakis, Oderda, Sterman, hep-ph/9803241

Agrees with known divergences:

Giele, Glover (1992); Kunszt, Signer, Trócsányi, hep-ph/9401294; Catani, hep-ph/9802439

finite, hard parts scheme-dependent!

Expansion of 1-loop amplitude

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 12

2-loop soft anomalous dim. matrix

• Classify web graphs according to number of eikonal lines (nE)

• 4E graphs factorize trivially into products of 1-loop graphs. • 1-loop counterterms cancel all 1/ poles, leave no contribution to

3E graphs are of two types

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 13

Triple-gluon-vertex 3E graph vanishes

Change variables to “light-cone” ones for A, B:

vanishes due to antisymmetry under

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 14

Other 3E graph factorizes

contains

The sum is color-symmetric, and factorizes into a product of 1-loop factors,

which allows its divergences to be completely cancelled by 1-loop counterterms

Same change of variables and transformation takes this factor to the one for the flipped graph:

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 15

The 2E graphsAll were previously analyzedfor the cusp anomalous dimension

Korchemsky, Radyushkin (1987); Korchemskaya, Korchemsky, hep-ph/9409446

Same analysis can be used here, although color flow is generically different, thanks to the identity – for non-color 1/ pole part of graphs:

All color factors become proportional to the one-loop ones,

Proportionality constant dictated by cusp anomalous dimension

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 16

Consistency with explicit 2-loop computations

• Results for

• Organized according to Catani, hep-ph/9802439

Anastasiou, Glover, Oleari, Tejeda-Yeomans (2001); Bern, De Freitas, LD (2001-2); Garland et al. (2002); Glover (2004); De Freitas, Bern (2004); Bern, LD, Kosower, hep-ph/0404293

looks like ???

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 17

2-loop consistency (cont.)

• Resolution is that scheme of Catani, hep-ph/9802439

is non-minimal in terms of 1/ poles• Color-nontrivial matrices are included in finite part• To compare to a minimal organization we have to commute two matrices:

• Then everything agrees Electroweak Sudakov logs agree with 2 2 results Jantzen, Kühn, Penin, Smirnov, hep-ph/0509157v3

Sept. 7, 2006 L. Dixon Two-Loop Soft Anomalous-Dim. Matrix 18

Proportionality at 3 loops?

Again classify web graphs according to number of eikonal lines (nE)

6E and 5E graphs factorize trivially into products of lower-loop graphs; no contribution to thanks to 2-loop result

4E graphs

also trivial???

and then there are more 4E graphs, and the 3E and 2E graphs…

use same (A,B) change of variables

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Implications for resummation

• To resum a generic hadronic event shape

requires diagonalizing the exponentiated soft anomalous dimension matrix in color space• Because of the proportionality relation, same diagonalization at one loop (NLL) still works at two loops (NNLL), and eigenvalue shift is trivial!• This result was foreshadowed in the bremsstrahlung (CMW) scheme Catani, Marchesini, Webber (1991)

for redefining the strength of parton showering using

Kidonakis, Oderda, Sterman, hep-ph/9801268, 9803241; Dasgupta, Salam, hep-ph/0104277; Bonciani, Catani, Mangano, Nason, hep-ph/0307035; Banfi, Salam, Zanderighi, hep-ph/0407287